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From: Beman Dawes (bdawes_at_[hidden])
Date: 2008-05-29 21:51:07

Neil Mayhew wrote:
> On 28/05/08 04:25 PM Beman Dawes wrote:
>> Neil Mayhew wrote:
>>> ... have a base class that contains everything except the
>>> constructors, and a derived class that adds the constructors
>> There are some issues with the base class approach. For it to be a
>> POD, it can't have constructors, base classes, or private members.
> Strictly speaking, yes, although we are only concerned about memory
> layout here, so the class doesn't have to be a true POD. The other
> concern is for the class to be able to live inside a union, and I think
> the constructors are the the only thing stopping that. (I #if'd the
> constructors and I was then able to use endian in a union.)

I was wrong above - you are correct that just the constructs have to be
removed. Private members and base classes are OK.

>> It might be better to just have two separate types, endian and
>> old_endian. (old_endian isn't very clear as a name, but calling it
>> pod_endian becomes invalid the moment C++0x ships.)
>>> The typedefs to big32_t etc. would then be #if'd to correspond to the
>>> POD or non-POD classes as desired. I think this is better than
>>> putting the #if's around the constructors themselves.
>> Hummm... Seems too obscure. Better to have two sets of typedefs, with
>> the C++03 set prefixed by old_ (or whatever better name anyone can
>> come up with.)
> I don't like "old_", at least not if it has to appear in my code! :-)
> I would like my client code to remain essentially the same when I
> upgrade to a C++0x compiler. This means having the same type names,
> which is why I suggested conditional typedefs. I thought this was
> cleaner than putting the conditionals inside the endian class itself.
> However, it doesn't make a lot of difference.
> Perhaps the simplest and best solution is therefore:
> class endian< ... >
> : cover_operators< ... >
> {
> public:
> #if defined(CXX_0X) || !defined(ENDIANS_IN_UNIONS)
> endian() {}
> endian(T val) { ... }
> #endif

That would work. The macro names need tweaks. Maybe


It's been a long day so that'll need checking in the morning:-)

Thanks for the ideas,


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